Microfluidic devices are becoming increasingly critical to biochemical analysis. These devices may have channels whose cross-sections are in the order of 1 .mu.m to 1000 .mu.m A fluid containing a sample to be analyzed and a reagent for activating the sample are delivered along channels to a reaction zone in the microchip. Pumping of the fluid is often carried out with external pumps, or electrical pumps that rely upon principles such as electroosmosis, electrophoresis and dielectrophoresis. When external pumps are used, problems can arise in both sample and reagent delivery.
For example, in sample delivery, transfer of sample to the chip may result in pressure differentials in excess of the pumping capacity, with resulting pressure fluctuations. In reagent delivery, the channels in the chip must be manually primed with reagent, with risk of cross-contamination.